Using Herbicides on Horse Farms

Weeds are a constant nuisance in pastures. They crowd out desirable forages; compete for sunlight, water, and nutrients; and may be toxic to livestock. Weed control is an important pasture management activity, but it is not as simple as reaching for an herbicide whenever weeds are present.

Weed control starts with proper care of the forages. This includes regular soil testing, liming and fertilizing, mowing, overseeding, and grazing management. When forages are healthy and thriving, they stand a better chance of competing with the weeds for resources.

Weeds can also be eliminated by mechanical means, such as hand-pulling, string trimming, and frequent, well-timed mowing. These methods tend to be quite effective on many annual weeds because they can prevent seed drop.

If the prior steps do not result in adequate weed control, then herbicides can be considered. They are a tool in the weed control toolbox. When used properly, they pose minimal risk to horses and humans. This article is a summary of herbicide basics, health risks, and how to use them safely for those who are inexperienced with them. Throughout the article, there are links to more detailed articles for more in-depth information on each topic.

Where trade names appear, no discrimination is intended, and no endorsement by Penn State Extension is implied.

Herbicide Basics

Herbicides are a subcategory of pesticides intended to kill plants or disrupt their growth. An herbicide product will have one or several active ingredients and some inert ingredients that do not have herbicidal activity but may improve the effectiveness or safety of the product. There can be several differently named commercial products with the same active ingredients or combinations of active ingredients. All commercial herbicides are required to be registered with the Environmental Protection Agency (EPA).

Herbicide active ingredients are typically classified as either non-selective (or broad-spectrum), meaning they affect many types of plants, or selective, meaning they only affect some types of plants. In a pasture situation, we usually choose selective herbicides that are effective on broadleaf weeds, but safe for established grasses. Broadleaf plants generally have wide or round leaves (e.g. dandelion, milkweed), as compared to the thinner, blade-shaped leaves of grasses and sedges. However, this does not mean that every plant with a wide leaf is a “broadleaf,” and every narrow leaf is a "grass." Before making herbicide decisions, it is essential to identify the weed properly so the right herbicide can be selected. Rarely, we may choose to use a non-selective herbicide on a pasture; however, we might spot-spray a small patch of weeds or may want to kill all the vegetation before a complete reestablishment of forages.

A patch of broadleaf weeds (mugwort) in a grass pasture. Photo by Laura Kenny.

Herbicide active ingredients may also have systemic or contact effects. Systemic herbicides are absorbed into the plant’s interior structures after being applied (usually to the leaves and stems) and transported throughout the entire plant, including the root system. Contact herbicides only damage the part of the plant to which the spray droplets adhere and often do not harm the root system. Therefore, even and thorough spray coverage of the plant is essential.

If you are not comfortable applying herbicides, you can hire a licensed pesticide applicator to spray them on your farm. You should still be familiar with the content in this article if you plan to hire an applicator.

Human Health Risks of Using Herbicides

By definition, all pesticides are toxic, because they are intended to damage living organisms. All EPA-registered pesticides (including herbicides) have undergone scientific testing to determine health risks to humans, animals, and the environment. These studies examine acute toxicity (a single exposure) and chronic toxicity (long-term exposure) to pesticide ingredients.

Acute toxicity refers to the health effects of a single exposure to the chemical, such as spilling it on your skin or splashing it in your eye. Every EPA-registered pesticide label will contain a "signal word" to indicate the acute toxicity level of its active ingredients based on scientific testing of oral, dermal, ocular, or inhalation toxicity. The word CAUTION signals a low-toxicity ingredient, WARNING signals moderate toxicity, and DANGER or DANGER/POISON signals high toxicity. For more information, read Toxicity of Pesticides.

Each of the herbicide labels in this photo has the signal word in large text. Photo by Penn State Pesticide Education Program. The label used for educational purposes only.

Chronic toxicity effects of an ingredient can be looked up on a document called a Safety Data Sheet or SDS. These are available online from the manufacturer of all registered pesticides. For more information and resources related to finding labels, read Pesticide Labels and SDSs.

The health risk involved with using pesticides depends on the toxicity of the product and the amount of exposure to the product. Consumers can reduce their health risk by using low-toxicity products (CAUTION signal word) and reducing exposure by protecting themselves when using the product. All pesticide labels contain instructions for required personal protective equipment (PPE) such as gloves, long sleeves, or respirators when using that specific product. Using the recommended PPE is how you reduce your possibility of exposure and, therefore, the health risk of using the product. Labels will state the "Minimum PPE;" however, you can always use more PPE than the minimum. Information related to protective equipment can be found in the Personal Protective Equipment (PPE) or Agricultural Use Requirements section of the herbicide label. For more information on PPE, read What You Need to Know about Protecting Yourself When Using Pesticides.

PPE requirements may be as simple as what you see in this photo: gloves, rubber boots, and long pants and sleeves. More toxic chemicals may require more PPE, such as a full-body suit and respirator. Photo by Penn State Pesticide Education Program.

Horse Health Risks of Using Herbicides

When choosing herbicides for a pasture, you must use a product that is labeled for grass pasture/grazed forage use. It may also be called "established pasture or rangeland." The label may also specify if the product can be used on cool-season vs. warm-season forages, and legumes vs. grass. This is a legal requirement, but it is also a safety measure. It means that the product has been tested for safety in grazing animals. It is not acceptable to use products labeled for turf or lawns on pastures. Even if the active ingredient is the same in a lawn product, the inert ingredients may be different and not EPA-approved for consumption by grazing animals. If you are uncertain if the product has the correct label for the intended forage, write down the EPA registration number and talk to an industry representative or Extension professional about the target crop before purchase. 

The label will state how long you need to keep animals off the pasture after applying the product (the grazing restriction). Most pasture herbicides have a zero-day grazing restriction for all livestock other than lactating dairy animals. It is not necessary, but some people will follow the lactating dairy grazing restriction for horses just to be extra cautious. We often recommend keeping horses off of treated pastures until the weeds have died, especially if some weeds are toxic. Wilting plants can become more palatable to animals and increase the likelihood of an equine plant poisoning.

Legal Considerations for Herbicides

When it comes to pesticide application, there is a simple saying: "The label is the law." This means that if you do not follow the instructions on the label or use a product off-label, then you are legally liable for any harm that may arise.

On this container of herbicide, you can see that the back label peels off and expands to provide all the relevant instructions. Photo by Penn State Pesticide Education Program. The label used for educational purposes only.

Pesticide regulations vary from state to state—products may be licensed for use in some states but not others. Sometimes, you must be a licensed pesticide applicator to purchase and apply a particular product (known as a "restricted use pesticide" or RUP), and in other states, anyone can legally buy and apply the same product as long as they follow the label directions ("general use pesticide"). Any time you want to use an herbicide, make sure that it is licensed for use in your state. You can look up herbicide labels and restrictions on CDMS and Greenbook herbicide databases.

Anyone who comes to your farm to spray pesticides, including herbicides, must have a commercial pesticide applicator's license in the state of Pennsylvania. You can spray on your own farm without a license as long as the product is not classified as restricted use by the PA Department of Environmental Protection, and you are following the label instructions precisely.

As mentioned previously, it is considered an off-label use (and therefore illegal) to apply an herbicide to a crop for which it is not labeled. Therefore, ensure that all herbicides used on your pastures are labeled for pastures or forages. The labels may further specify which forages can be treated (cool-season or warm-season, grass or legume, specific forage species).

For more information, read Pesticide Laws and Regulations.

Choosing an Herbicide and Reading the Label

You can call your local Extension office for advice to select an herbicide for your particular weed problem. You can also consult the Penn State Agronomy Guide if you live in Pennsylvania, or the Mid-Atlantic Field Crop Weed Management Guide in the mid-Atlantic region. In these publications, you will find tables showing the efficacy of many pasture herbicides on common pasture weeds. There are also notes from the labels of each listed herbicide which include application rates, any residue or withdrawal concerns, and recommendations for application timing.

In this photo you can see page 1 of the label on the herbicide container. You can peel it back to read the rest of the instructions. Photo by Penn State Pesticide Education Program. The label used for educational purposes only.

Before purchasing an herbicide, you should look up a current version of its label on the manufacturer’s website or another source like the CDMS database. The label specifies some very important information, such as active ingredients, toxicity signal words, which crops the product is registered for, weeds controlled by the product, what PPE is required, restrictions for grazing, cutting hay, reseeding, and residue like manure, how much product to use per acre and in what spray volume per acre or area, and how it should be mixed; as well as any necessary spray additives (or adjuvants) that need to be added to the spray tank to optimize the herbicide’s performance. Read more about labels in What You Need to Know About Reading a Pesticide Label.

It is important to realize that product application rates are quite specific; more is not better. Additionally, many agricultural herbicide products are not ready to use; they need to be measured and mixed with water (i.e., the carrier) and other additives in your sprayer. Depending on the herbicide, measurements can be based on either volume or weight; therefore, you must have a designated and accurate measuring scale and/or graduated measuring container before preparing for an application.

It is important to have a measuring cup or graduated cylinder with the appropriate measurements that is only used for pesticides. Photo by Penn State Pesticide Education Program.

If you do purchase a granular or powder formulation herbicide, most often they are formulated to be mixed in water, not applied as a solid directly from the package. Again, this stresses the importance of reading the label prior to application to determine how to apply the product.

The label is the law, so make sure you understand all the instructions and can carry them out. You should also double-check the label on the actual product that you receive, as there may be slight variations from the one you looked up online. If you don't feel confident that you can understand or follow the instructions, it might be best to hire a licensed applicator.

How to Apply Herbicides

Using herbicides is a personal choice for any farm owner. They should not be seen as a "quick fix," as there are many factors that can contribute to weed growth. If you decide to use an herbicide, you’ll need to know the steps to apply it and how to avoid causing effects that you didn’t intend.

Pasture herbicide applicators come in two general application types: you can either use a single-nozzle spot sprayer/backpack/hand-held sprayer for small areas, or you can use a broadcast (multiple nozzle, boom) sprayer that must be mounted onto a tractor or farm utility vehicle to cover larger acreage.

This is an example of a boom sprayer used on a golf course. Smaller tanks and sprayers can be mounted onto farm utility vehicles. Photo by Penn State Pesticide Education Program.

Both types of sprayers should be calibrated using water to ensure you know the rate (i.e., spray volume) you are spraying (gallons per acre) before you begin. Be careful when applying herbicides near waterways and sensitive plants (such as your neighbor's soybean field, vineyard, orchard, your garden, etc.) because small herbicide droplets and/or vapor can drift or move off-target, especially under certain weather conditions such as wind, high heat or temperature inversions. Some herbicides are more prone to drifting than others (more information in "Important Cautions"). The label will provide instructions to reduce drift. If you are not comfortable applying herbicides, you can hire a licensed pesticide applicator to do it for you.

Important Cautions

There are a few extra challenges you should be aware of when using broadleaf herbicides on pastures.

• Most broadleaf herbicides will injure clover (and other legumes). This may be desirable for some, but if you just planted a pasture seed mix with clover, you may not want to kill it.
• If you eliminate a patch of weeds, you are left with bare ground. This presents an erosion risk. There will also be other weed seeds in the soil that may germinate, replacing one weed problem with another. Soil testing and reseeding should be part of your weed management plan.
• Most herbicides that are safe for established grasses can injure young grass seedlings or prevent newly planted grass seed from germinating. Each product label will specify how long you need to wait after seeding to spray the herbicide, and how long after spraying you can plant grass seed. In some cases, the herbicide remains active in the soil for months, which is helpful for weed control but problematic if you planned to reseed right away. Read your label carefully and schedule your pasture renovation accordingly.
• One reason we recommend using herbicides prudently is that some weeds are developing resistance to herbicides. This means that the chemical will be less effective on that plant than it once was. One example in Pennsylvania is marestail (Erigeron canadensis, aka horseweed), which has developed resistance to two commonly used herbicide ingredients.

Herbicide-resistant marestail in a wheat field. Photo by Penn State Pesticide Education Program.

• "Herbicide spray drift" is when herbicides move in the air during or after an application, potentially causing damage to nearby susceptible plants. Spray drift can travel feet to miles. Common treatments like 2,4-D and dicamba are particularly susceptible to volatilization and drift. There are specific precautions that applicators can take to reduce spray drift, like spraying at cooler temperatures (less than 80°F), using nozzles that produce larger droplets, and monitoring wind speeds. Be aware of your proximity to other crops, orchards, vineyards, and even personal gardens or landscaping when using herbicides.

Manure and Plant Residue Issues

There are some herbicides that are so safe for grazing animals that when the animal grazes on sprayed forage, the herbicide will pass through the digestive system totally unchanged. Unfortunately, that means that the animal’s manure now has herbicidal action! One common chemical that does this is called aminopyralid, present in pasture products such as GrazonNext, Milestone, and Chaparral. However, products containing the ingredients picloram and clopyralid also have "residue restrictions" on their labels.

If using a product with these ingredients on a pasture, it is critical to ensure that manure from animals grazing that pasture is not applied to crops that are sensitive to the chemical (most broadleaf crops). It also cannot be applied to home gardens or flower beds. Composting the manure does not inactivate the herbicide. The herbicide can remain active for up to 18 months!

If using these herbicides on hay fields, some labels state that the treated hay cannot leave the farm. The herbicide may still be active in the hay, therefore the livestock will have contaminated manure. It is imperative that the person who feeds this hay knows about the manure restrictions.

While these herbicides are quite effective on a wide range of broadleaf weeds, applicators must read and follow all of the restrictions on the label.

This herbicide contains aminopyralid, and its label includes specific precautions regarding harvested forage and manure that has been exposed to this product. Photo by Penn State Pesticide Education Program. The label is used for educational purposes only.

Alternative Herbicides and "Home Remedies"

Many farm owners are interested in alternative "natural" options for weed control. Unfortunately, many of these natural products or home remedies do not have scientific testing behind them to prove their efficacy or safety. There are also some potential problems with home remedies, such as:

• Many home remedies (e.g., vinegar) will burn the top growth but not affect the roots. Therefore, the weed will likely grow back.
• Home remedies have not been tested for safety for humans, grazing animals, or the environment. There is no label specifying how to protect yourself or at what rate to use it.
• Some home remedies make the soil inhospitable to plant growth. Using things like salt or diesel may kill some weeds, but then nothing else will grow there! This is not desirable in a pasture.

If a product does not have an EPA Registration Number on the label, you should not use it. In addition, natural or organic pesticides are not necessarily safer than synthetic chemicals; read the label and note the toxicity signal word.

Example: The organic herbicide horticultural vinegar (acetic acid) can be effective as a non-selective herbicide, but it is much stronger than food-grade vinegar and extremely caustic. It is quite dangerous to handle without proper PPE; it carries the signal word DANGER. To be used legally as an herbicide, the product must be registered as a pesticide in your state and have a label with instructions for use.

Even organic products are required to have an EPA registration number. Photo by Penn State Pesticide Education Program. The label is used for educational purposes only.

Conclusion

Herbicides, when used properly, can be an effective and important tool for pasture managers. However, they present inherent risks and should only be applied by people who understand how to do so safely. The level of risk depends on the toxicity of the product (signal word) and the amount of exposure to the product, so wearing PPE according to the label is imperative. In fact, all instructions listed on the label are legal obligations. The label is the law. Herbicides labeled for pasture and forage use have been tested for safety with grazing animals. Most pasture herbicides are formulated to be applied in a liquid solution using a sprayer, and many are not ready to use out of the container. Be aware of precautions on the label, such as herbicide drift and residue restrictions. If in doubt, contact your local Extension office, a professional pesticide applicator, or the product manufacturer to ask questions.

Further Reading

Introduction to Weeds and Herbicides

Where trade names appear, no discrimination is intended, and no endorsement by Penn State Extension is implied.